1. Field of Use
The invention relate to the use of cement to stabilize substrate soils and, more particularly, to the use of a novel cement slurry that remains in suspension and does not prematurely set.
2. Related Art
In the construction industry, a stable rigid base is required for paving, building and parking structures, which requires the stabilization of the substrate soil. This stabilization may be accomplished by combining cement with the soil. Combinations of cement and soil are referred to as, but not limited to, soil cement, cement treated base, cement stabilized soil, and cement treated soil. The creation of soil cement involves the addition of specified amounts of cement per cubic unit of soil. The aforementioned soil is then graded and compacted to meet requirements specified in job plans and drawings. The cement treated soils are then allowed to cure, a chemical process whereby the cohesive material gains in strength and rigidity over time.
One method commonly used to create cement treated soil is a dry cement method. This generally involves using a pneumatic pumping apparatus to blow dry cement onto a soil substrate and then mixing the cement into the soil. While commonly used, this process is considered undesirable due to the large amount of dust created by the blowing of fine cement powder. This is especially undesirable for the creation of soil cement in more populated areas.
Another method to create soil cement is to use a cement slurry. A cement slurry is basically powdered cement suspended in water. There are various known methods for creating cement slurry. The cement slurry preparation is placed over a substrate soil and then mixed in using mechanical means. Cement slurries are seen as a more desirable method of cement treatment than that of dry cement methods. However, slurry methods have proven to be very problematic in use. Cement slurry will harden in shipping vehicles if not removed in a timely manner. In addition, the cement itself will separate or fall out of solution almost immediately after mixing with water. Even in concentrations as low as 10% cement in water, the cement will begin to fall out of solution within a couple of minutes. As a result, cement slurries have been effectively limited to 30% cement in water, even when using mechanical mixing. In addition, the excess water required to keep the cement in suspension often raises the moisture level of the substrate soil above the desired range for creation of the soil cement.
The use of chemical retardation to prevent the set of cement based materials, including cement slurry, is well known throughout the industry. One common retarding compound is sugar. Employing chemical retardation in cement slurry tends to diminish the problems of setting prior to application. However, it additionally tends to increase the rate at which the cement falls out of solution.
Methods to overcome the propensity of cement to precipitate inside transportation vehicles include drum mixing and recirculation pumps. Both methods involve mechanical means to keep the solution moving so as to prevent the cement from settling within the shipping vehicle. Should settlement occur, the resulting segregation of the water and cement particles make it impossible to properly regulate the dispersion of the binary solution. This event also creates a great deal of maintenance problems for the vehicle operator. As a result of these limitations and despite the lack of dust creation, the use of cement slurry tends to be rare. The problematic properties of cement slurry have significantly limited its use and made other stabilization methods more desirable. Consequently, there remains a need for a cement slurry that does not prematurely set or settle out during transport.
As an alternative to soil cement, a process called full depth reclamation can be used to provide a base for structures such as roads, parking lots, and other paved areas. This process involves grinding up and pulverizing the asphalt surface and blending it with the underlying base, subbase, and/or subgrade material. Cement and water is added to the combined materials to stabilize it much in the same way that cement can be added to substrate soil to created stabilized soil cement. The mixture is then compacted in place to form a stabilized substrate for the new paving. This process allows deterioriated roadways and other paved surfaces to be rebuilt without requiring lots of additional aggregate raw materials or having to cart away and dispose of the old asphalt and other materials. The process also allows roadways to be repaved and strengthened without changing the level of the grade, thereby avoiding issues where the pavement meets curbs and drain grates as well as avoiding changing the amount of clearance under overpasses on roadways. However, this process, because it involves the addition of cement to stabilize the base, runs into the same problems discussed above with respect to the application of cement for soil stabilization.
While rare, thixotropic and other thickeners have been occasionally used in connection with cement applications, although not in connection with cement slurries. Methylcellulose compounds have been used when pouring concrete underwater. In contrast to thickening the concrete, the methylcellulose is added to minimizes the amount of concrete that is washed away by the surrounding water. This reduces the requirement that the structures be over engineered to account for the amount of sacrificial concrete that is washed away. Starch or other thickeners such as silica have been used in specific cement applications such as shotcrete, where dry or mixed concrete is spayed onto walls and/or ceilings, and in thin set mortar. However, in these applications, the thickener is added to a mixture that is already very low in water content and viscosity, for purposes of increased self-adhesion, and does not lower the viscosity of a fluid, reduce the tendency for disaggregation or settling out of suspended elements in a fluid, or increase the time before the mixture sets.